The current supplied to cells used in electrochemical plants, especially in plants of metal electrodeposition such as metal electrowinning or electrorefining may be apportioned in a very diverse manner to the various electrodes installed, with negative consequences on production. This phenomenon can occur for several reasons. For example, in the particular case of metal electrowinning or electrorefining plants, the electrodes of negative polarity (cathodes) are frequently removed from their seats to allow harvesting of the product deposited thereon, to be later put back in place for the following production cycle. This frequent handling, generally carried out on a very high number of cathodes, often leads to an imperfect repositioning on the relative bus-bars and to less than ideal electrical contacts, which can also be occasioned by fouling deposited in the receiving seats. It is also possible that the deposition of the product takes place in an irregular manner on the electrode, with formation of mass transport gradients altering the profile of the cathode surface. When this occurs, an electrical imbalance is established due to the fact that the anode-to-cathode gap is no longer constant along the whole electrode surface: the electrical resistance, being a function of the distance between each pair of anodes and cathodes, becomes variable, worsening the problem of irregular current distribution.
The current, therefore, might be apportioned in different amounts to each electrode due both to bad electrical contacts between the latter and the bus-bars and because of alterations of the surface profile of the cathodes. Moreover, even the simple wear of the anodes can affect the distribution of current.
These inhomogeneities in the distribution of current can lead to anode-to-cathode short-circuits. Another frequent cause of short-circuits, particularly in the case of copper electrodeposition, is the occasional formation of dendritic deposits that grow locally at faster rate as long as the local anode-to-cathode gap decreases, with an increasing fraction of current that concentrates at the point of growth of the dendrite, until the onset of a short-circuit condition between the cathode and the anode occurs. In case of short-circuit, the current tends to concentrate on the shorted cathode, subtracting current to the remaining cathodes and seriously hampering the production, which cannot be resumed until the shorted cathode is disconnected.
An uneven current distribution, besides generating a loss of quality and production capacity, as indicated above, puts at risk the integrity of advanced anodes obtained from titanium meshes, shortening their lifetime.
In industrial plants, given the high number of cells and electrodes present, the task of detecting irregularities in the distribution of current is very complex. Such detection, in fact, involves thousands of manual measurements performed by operators via infrared or magnetic detectors. In the specific case of metal electrowinning and electrorefining plants, these detections are carried out by the operator in a high temperature environment and in the presence of acid mists, mainly containing sulphuric acid.
Moreover, conventional manual elements used by operators, such as gaussmeters or instruments with infrared sensors, allow to locate only large imbalances of current distribution, since they actually detect imbalances associated with changes in the magnetic field or temperature.
These manual or semi-manual systems have the disadvantage of being unsuitable for continuous operation (only allowing spot checks), very expensive and potentially hazardous for the operator's health.
There are known systems for wireless monitoring of the cells which, although being permanent and working in continuous, only detect changes in voltage and temperature for each cell and not for every single electrode. This information, as explained above, is imprecise and overall insufficient.
An attempt to overcome the above problems is disclosed for example in WO2013037899. The invention described in such patent application has the disadvantage of entailing the fixing of thousands of contacts directly on the bus-bars, a complicated task to accomplish in a plant during operation. Furthermore, such indirect current measurement requires the use of a complicated calculation model that needs to allow for several approximations.
For these reasons, there is a need expressed by the industry to get hold of a technically and economically feasible system for permanently and continuously monitoring and measuring current distribution in each and every electrode installed in the cells of a metal electrodeposition plant.